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The military trial against US Army Brig Gen Jeffrey Sinclair is a rare sexual assault prosecution against a senior US military officer

A US Army general accused of sexual assault has pleaded guilty to three lesser charges in a move his lawyer says will help his chances at trial.

Brig Gen Jeffrey Sinclair is accused of forcing a female captain to perform a sex act on him and threatening to kill her family if she reported it.

On Thursday he pleaded guilty to adultery and other charges.

His lawyer hopes the plea will limit the evidence prosecutors can present on the outstanding charges.

The announcement delayed opening statements in the case, previously scheduled for Thursday morning.

Gen Sinclair, the former deputy commander of the elite 82nd Airborne Division, is believed to be the most senior member of the US military to face sexual assault charges.

His trial in Fort Bragg, North Carolina, comes as the US defence department struggles with what critics have described as an epidemic of sexual assault and misconduct cases within military ranks.

The US Senate is weighing legislation that would remove decisions about the prosecution of sex crimes from the military chain of command.

'Pathetically weak'

On Thursday, Gen Sinclair pleaded guilty to improper relationships with two female Army officers, violating orders by possessing pornography in Afghanistan, and conduct unbecoming of an officer.

In an interview with the Associated Press news agency, his lawyer Richard Scheff said his client was taking responsibility for his actions but also strengthening his legal position.

"The government now has a big problem," Mr Scheff said.

"It took pathetically weak assault charges and put a fancy wrapper around them. We just tore the wrapper off. The prosecution team no longer gets to distract us with salacious details about acts that aren't even criminal in the civilian world."

The five remaining charges include sexual assault, threats against the captain's family, and charges that he used his superior rank to force the continuation of their affair. The general could be imprisoned for life if he is found guilty of the most serious charges.

His defence plans to present evidence at trial that the female captain lied under oath during a pre-trial hearing in January about her handling of an old iPhone containing messages between her and the general, arguing she made the allegations after the general refused to leave his wife.